1. Field of the Invention
The present invention relates to an OFDM (Orthogonal Frequency Division Multiplexing) transmission method, and more specifically to a scheme for transmitting data via a wired or wireless transmission path by using an OFDM signal, and a transmitter/receiver therefor.
2. Description of the Background Art
In an OFDM signal transmission scheme, a demodulation characteristic deteriorates due to any distortion observed in a transmission path, out-of-synchronization after passage of time, frequency drift between a transmission side and a reception side, amplitude and phase errors resulted from phase noise in a local oscillator provided in a receiver, and the like. Such error factors provoking the demodulation characteristic to deteriorate are hereinafter referred to as frequency response variation.
In the OFDM signal transmission scheme, for synchronization with a receiver, a transmitter often inserts one or more preambles into a signal before data transmission. The preamble is longer than one symbol time wise, and therewith, a frequency response of the transmission path can be correctly estimated. The more preambles lead to the higher accuracy in estimating the frequency response, but the transmission speed shows a considerable drop.
Therefore, as is disclosed in Japanese Patent Laying-Open No. 8-265293 (96-265293), interposing one or more pilot carriers in between data carriers in a data symbol is conventionally popular.
The OFDM signal is structured by a plurality of equal-length symbols, each of which includes several subcarriers. The above-mentioned data carrier and pilot carrier are both subcarriers. In the above prior art, a phase error of the pilot carriers included in the data symbol is detected for every data symbol for compensation.
Such prior art, however, bares a problem in an environment where any higher-level noise is observed in the transmission path or a multi-path fading environment. Accordingly, the fewer number of pilot carriers per symbol, the lower the accuracy of phase error detection becomes. Although the more number of pilot carriers surely achieve the higher accuracy thereof, the occupied frequency bandwidth becomes wider, and the transmission speed considerably drops. Furthermore, it is also difficult to compensate for the amplitude error caused by any distortion observed in the transmission path.